The present invention relates to a semiconductor laser, more particularly to a semiconductor laser having a high-reflectivity-coating formed on its rear facet and a manufacturing method of the same.
For the purpose of increasing output powers from semiconductor lasers and/or improving temperature characteristics thereof, high-reflectivity-coatings with multilayers on the rear facets of semiconductor lasers are broadly used.
A conventional semiconductor laser having a rear facet on which a multi-layer high-reflectivity-coating is made is shown in FIG. 1. In this semiconductor laser, a first low refractivity film 10 is formed on a front facet 1 or a facet from which a laser beam is emitted, of the semiconductor laser 21. The film thickness of the first low refractivity film 10 is controlled so that it exhibits a reflectivity of about 30% or less. On the rear facet 2 opposite to the facet 1, a second low refractivity film 14 and a high refractivity film 17 are alternately formed, resulting in formation of the high reflectivity coating. The thicknesses and the number of the films 14 and 17 are designed so that the whole structure constituted by the films 14 and 17 alternately stacked exhibits a desired reflectivity ranging from 30 to 100%. The thicknesses of the second low refractivity film 14 and the high refractivity film 17 are often set so that their optical thicknesses are xcex/4 relative to a wavelength xcex of the semiconductor laser. With respect to a third low refractivity film 19 formed on the rear facet 2 side, as the uppermost layer, in order to obtain a desired reflectivity, an optical thickness of the film 19 is set to xcex/4, xcex/2 or an intermediate value between these values. The optical thickness is a product of its exact film thickness (physical film thickness) and refractivity when the incident angle of laser beam on the film is 0 degree.
In a red semiconductor with an oscillation wavelength ranging from 600 to 700 nm and a infrared semiconductor laser with an oscillation wavelength ranging from 700 to 800 nm, aluminium oxide Al3O2 exhibiting a refractivity ranging 1.6 to 1.7, silicon dioxide SiO2 exhibiting a refractivity ranging from 1.4 to 1.5, and the like have been used as the materials of the first to third low refractivity film. As the material of the high refractivity film, silicon nitride SiNx exhibiting a refractivity ranging from 1.8 to 2.2, amorphous silicon or xcex1-Si exhibiting a refractivity ranging from 3 to 5, titanium oxide TiOx exhibiting a refractivity ranging from 1.9 to 2.5, zirconium oxide ZrOx exhibiting a refractivity ranging from 1.8 to 2.2, and the like have been used.
As a difference between the refractivities of the low refractivity films and that of the high refractivity films is larger, a reflectivity per a pair of the low and high refractivity films is larger. Accordingly, it is desirable that a refractivity of the low refractivity film is as low as possible and a refractivity of the high refractivity film is as high as possible.
With respect to the low refractivity film, because a refractivity of SiO2 is lower than that of Al2O3, SiO2 is more desirable film material than Al2O3 in respect of the refractivity. However, while a linear expansion coefficient of silicon dioxide is about 1xc3x9710xe2x88x926 (1/K) or less, single crystal substrates of compound semiconductors such as AlGaAs group, InP group, InGaP/InGaAlP group and GaN group, which are often used for the semiconductor laser material, that is, such as gallium arsenide (GaAs) substrate, phosphorus (InP) substrate, and sapphire (Al2O3) substrate exhibit a linear expansion coefficient ranging from 4 to 7xc3x9710xe2x88x926 (1/K). The linear expansion coefficient of SiO2 and that of these substrates are greatly differ. SiO2 films was deposited at high temperatures exceeding 100 degrees centigrade and the laser chips with high-reflectivity-coatings were die-bonded on heatsink at a high temperatures exceeding 200 degree centigrade. The great difference between SiO2 films and a rear facet of the semiconductor laser enhances a large stress and causes problems the laser characteristics or laser reliability. For example, a SiO2 film is partially peeled off from the rear facet by the stress thereof. Accordingly, characteristics or reliability of the laser are degraded. For this reason, SiO2 has been seldom selected as the low refractivity film, and Al2O3 exhibiting a linear expansion coefficient 7xc3x9710xe2x88x926 (1/K) which is closer to that of the compound semiconductors has been often used.
However, since Al2O3 hardly creates a refractivity difference from the high refractivity film 17, any of countermeasures of increasing the number of the pair of low and high refractivity films and using of higher refractivity material for the high refractivity film must be selected. There are following problems in both countermeasures.
In the case of increasing the number of the layers, the stress between the low/high refractivity film and the compound semiconductor layers grown on the semiconductor substrate, as well as between the low/high refractivity film and the semiconductor substrate, as well as between two of the low/high refractivity films, increases, resulting in the film peeled off, not being able to used as a passivation film, promoting a device degradation because the stress is applied to the rear facet. Thus, it is happened a problem of reliability of the semiconductor laser. It is considered that the number of the layers that creates no problem of the reliability of the compound semiconductor is 10 or less. However, when Al2O3 is selected as the low refractivity film and SiNx is selected as the high refractivity film, the films of about 15 layers must be coated to obtain the refractivity of 90% or more, so that a problem of reliability is posed. Moreover, even if films posing no problem of the reliability are selected, a time required for depositing the films is too long, so that a problem of productivity is posed.
Next, as the high refractivity film, a selection of a film exhibiting a higher refractivity, for example, of xcex1-silicon film, is conceived. After Al2O3 having a optical thickness xcex/4 is employed as the low refractivity film, xcex1-silicon having a optical thickness-xcex/4 is employed as the high refractivity film, a theoretical calculation is performed regardless an optical absorption, so that the reflectivity of about 90% can be realized by the number of the layer equal to 9 or less. In fact, similar layer structures have been broadly used in upper-output-power lasers.
However, since xcex1-silicon exhibits a high optical absorption of the order of 1xc3x97104cmxe2x88x921 in a 600 nm band of the laser wavelength, the following problems are posed. First, it is difficult to obtain a higher reflectivity because of the optical absorption. When a theoretical estimation is conducted for the case where a design is done so as to obtain a reflectivity of around 90% in a 600 nm band of the wavelength, the reflectivity is lowered by about 10%, compared to the case where the optical absorption is assumed to be zero in the xcex1-silicon film. Although the reflectivity increases for increasing the number of the layers in the pair of the low and high refractivity films, it is difficult for the reflectivity to exceed 90%. Also, since a part of an output power from the laser is absorbed by optical absorption in the xcex1-Si film, the output power is reduced. Accordingly, when it is tried to obtain a desirable light intensity, a threshold current (Ich) of the semiconductor laser and an operation current (Icp) thereof increase. Therefore, it is always happened that an efficiency improvement of the laser cannot be taken in spite of formation of multi-layered high reflectivity coating for increasing the output power of laser and improving laser characteristics at high temperatures.
It is pointed out that it is capable of being lowered a COD (Catastrophic Optical Damage) level by a light absorption in xcex1-Si film. (Kiyotake Tanake et. al., Paper being prepared for publication in 43-th associated meeting relating to Japanese Applied Physics Society, 26a-C-7, 1996). The COD is an irreversible degradation around a laser facet of semiconductor lasers, and has limited by the optical output power from semiconductor laser. An optical absorption occurs near the facet because the energy band gap of the active layer near the surface is slightly small compared to the band gap inside of the laser. A heating-up is enhanced according to optical absorption, and the photochemistry reaction and the surface recombination at the facet are accelerated by the heating-up, and further heating-up is generated. There is a positive feedback loop that the energy band gap near the facet becomes still smaller by the heating-up and optical absorption becomes still larger. As a result, the facet is irreversibly damaged at an output power accelerated by the positive feedback. After the enhancement of the COD in the semiconductor laser, Ith increases remarkably compared with that before COD, or a laser stops oscillating. After generation of the COD, a non beam-emitted portion called a dark spot or a dark line were observed in a near field-electro-luminescence pattern from the degraded facet by applying a very low forward current to the laser and observing the light emitting area by using a microscope.
By this method, it can be confirmed that a degraded portion does not exist inside of the device, but near the facet.
It was thought that the COD is occurred around the front facet with a lower reflectivity coating thereon having a reflectivity, for example, 30% or less, because it is considered that the optical power density around the front facet is greater than that around the rear facet. However, the inventors have found out that a COD occurs around the rear facet with a high reflectivity coating including xcex1-Si films by optical absorption in xcex1-Si films. After an excessive currents are applied to five semiconductor lasers including the front facets with low reflectivity coatings having a reflectivity less than 30%, and the rear facets with the high reflectivity coatings including xcex1-Si films, and let degraded. The electro-luminescence patterns from the front facets were normal, but dark spots or dark lines were observed in those from the rear facets.
Furthermore, there were not any abnormal portions on the coatings observing a secondary electron microscope or an SEM images. Tanaka et al. told that they observed a destruction in a high reflectivity coatings and the COD were to be enhanced by the destruction in the above mentioned reference. However, such a phenomenon was not found as long as Inventors confirm.
As a cause which such the phenomenon generates, Inventors assume the following mechanisms. First, heating-up was enhanced by optical absorptions in xcex1-Si films in high reflectivity coatings. The energy band gaps of the active layers near the rear facets were shrinked by the heating-up. The band gap shrinkage causes not only the increases of the optical absorption, but also further heating-up. The optical absorption and the generation of heat organize a positive feedback loop, and the positive feedback loop enhances the COD""s at some output powers.
As described above, in the lasers with high reflectivity coatings including high refractive films with large optical absorption coefficients, the COD occurs around the rear facet but not occurs around the front facet. This causes the difficulties of increasing the output power from the lasers having the high reflectivity coatings on the rear facets including high refractive films with large optical absorption coefficients.
Moreover, by increasing the number of layer constructing a laser, a light intensity from the rear facet covered by the high reflective coating is decreasing. For regulating the output power of laser, it is common that an automatic power control or an APC feeding back the amount of laser to an output power from the rear facet by detecting photo-current in a rear photodiode. Since the output power from the front facet of the laser is proportional to the photo-current, the output power from the front facet can be controlled by a negative feedback of the photocurrent by a circuitry. However, using the xcex1-Si as the high refractivity film in multi-layered structure including over five layers, it is not capable of receiving an amount of light enough to control the power from the rear facet.
When TiOx or ZrOx with a very low absorption coefficient is selected as the high refractivity film, a problem relating to the foregoing points is not so much. Since these materials prepared by some deposition schemes have the optical absorption coefficients of zero or a little, there were less problem than the case using xcex1-silicon. The refractivity of these films may exceed 2.0, and a desired reflectivity can sometimes realize with the number of layers equal to 10 or less.
However, these films involve problems in their deposition methods. Deposition by electron beam heating is generally used for formation the TiOx or ZrOx film. In the film formation by the electron beam deposition, the deposited substance having high energy directly collides with the facet of the semiconductor laser, so that damages to the facet is happened. Actually, with respect to a red semiconductor laser, the laser using a film formed by the electron beam heating deposition tends to be degraded rapidly.
For this reason, a method to form a TiOx film by an ECR (Electron Cyclotron Resonance) sputtering which applies little damage to the facet is proposed, in Japanese Patent Laid-Open No. 9-162496 and in the paper being prepared for publication in 44th associated meeting relating to Japanese Applied Physics Society, 31-NG-7, 1997, by Kiyotake Tanaka et. al.
However, a target for manufacturing these films is too expensive. Moreover, Ti, that is the target material for formation of the TiOx film, strongly acts as an oxide getter, so that it is considered that use of Ti in the ECR sputtering in which the target reacts with gas entails difficulty. Accordingly, this method is not practical.
As described above, it is impossible to provide the conventional semiconductor laser in which the high reflectivity coating composed of multi-layers is coated on its facet, which is capable of preventing the occurrence of the problem peeling off the high reflectivity coating from its facet and the problem of the reduction in the reflectivity due to the optical absorption, in addition to the problem in the manufacturing processes.
The present invention was made to eliminated the above-described problems, and the object of the present invention is to provide a semiconductor laser with a multilayer high reflectivity coating, which is capable of exhibiting a high performance and high productivity such as a high reliability, a low threshold and a low operation current.
To achieve the above object, the present invention provide a semiconductor laser comprising a laser device having a first front facet emitting laser beam therefrom, and a second end surface, wherein the first rear facet is opposed to the second end surface, and a multi-layered high reflection film formed on the second facet, and including, a first low refractivity film formed on the second rear facet of the semiconductor laser, made by an apparatus for forming a thin film without damaging the film, and having a coefficient of linear expansion is 30% more/less than that of a substrate of the semiconductor laser, a refractivity n1 less than 1.8, and an optical thickness thereof about xcex/4 while xcex is defined as a wavelength of the semiconductor laser, a first high refractivity film formed on the first low refractivity film, made by an apparatus for forming a thin film without damaging the film, and having a refractivity n2 more than 1.9, an amount of k of a negative imaginary part of the refractivity or xe2x88x92ik less than 0.05, and an optical thickness thereof about xcex/4, and a second low refractivity film formed on the first high refractivity film, made by an apparatus for forming a thin film without damaging the film, and having a refractivity n3 less than 1.7 and less than n1, wherein the second low refractivity film is made by a material different from a material of the first low refractivity film.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.